Direct Band Gap Germanium Microdisks Obtained with Silicon Nitride Stressor Layers

Germanium is an ideal candidate to achieve a monolithically integrated laser source on silicon. Unfortunately bulk germanium is an indirect band gap semiconductor. Here, we demonstrate that a thick germanium layer can be transformed from an indirect into a direct band gap semiconductor by using silicon nitride stressor layers. We achieve 1.75% (1.67%) biaxial tensile strain in 6 (9) mu m diameter microdisks as measured from photoluminescence. The modeling of the photoluminescence amplitude vs temperature indicates that the zone-center Gamma valley has the same energy as the L valley for a 9 mu m diameter strained microdisk and is even less for the 6 mu m diameter microdisk, thus demonstrating that a direct band gap is indeed obtained. We deduce that the crossover in germanium from indirect to direct gap occurs for a 1.67% +/- 0.05% biaxial strain at room temperature, the value of this parameter varying between 1.55% and 2% in the literature.